Stabilized platinum catalyst

ABSTRACT

A stabilized platinum nanoparticle has a core portion surrounded by a plurality of outer surfaces. The outer surfaces include terrace regions formed of platinum atoms, and edge and corner regions formed of atoms from a second metal. The stabilized nanoparticle may be formed by combining a platinum nanoparticle with a metal salt in a solution. Ions of the second metal react with platinum and replace platinum atoms on the nanoparticle. Platinum atoms from the edge and corner regions react with the second metal ions quicker than surface atoms from the terraces, due to a greater difference in electrode potential between the platinum atoms at the edge and corner regions, as compared to the second metal in the solution. The platinum nanoparticle may include surface defects, such as steps and kinks, which may also be replaced with atoms of the second metal. In an exemplary embodiment, the platinum nanoparticle is a cathode catalyst in an electro-chemical cell.

BACKGROUND

The present disclosure relates to platinum nanoparticles. Moreparticularly, the present disclosure relates to stabilized platinumnanoparticles used as a catalyst in a fuel cell.

Platinum nanoparticles are well known for use as an electrocatalyst,particularly in fuel cells used to produce electrical energy. Forexample, in a hydrogen fuel cell, a platinum catalyst is used to oxidizehydrogen gas into protons and electrons at the anode of the fuel cell.At the cathode of the fuel cell, the platinum catalyst triggers theoxygen reduction reaction (ORR), leading to formation of water. The ORRreaction takes place at high potential, which makes the platinumnanoparticles unstable on the cathode, resulting in a loss inelectrochemical surface area of the nanoparticles. Due to potentialcycling during fuel cell operation, the platinum nanoparticles maydissolve. The atoms at the corners and the edges of the nanoparticleshave a higher surface energy and, as such, are more reactive thansurface atoms on the terraces of the nanoparticles. The nanoparticlescommonly include surface features or defects that form on the surfaceduring synthesis of the nanoparticles. The atoms that form these surfacedefects, including steps and kinks, are also more reactive sites on thenanoparticle, compared to the surface atoms on the terraces. The morereactive atoms are more prone to dissolving and forming oxides, ascompared to atoms having lower surface energy.

Although platinum is a preferred material for use as a catalyst in afuel cell, platinum is expensive. Moreover, the instability of theplatinum nanoparticles in the cathode environment results in a loss ofsurface area of the nanoparticles, and consequently a loss in fuel cellperformance. This requires a larger amount of platinum catalyst to beused in the fuel cell, which increases cost. There is a need for aplatinum nanoparticle that is more stable during operation as a cathodecatalyst in a fuel cell.

SUMMARY

A stabilized platinum nanoparticle is described herein and includes acore portion surrounded by a plurality of outer surfaces. The outersurfaces include flats or terrace regions formed of platinum atoms, andedge and corner regions formed of atoms from a second metal. Thestabilized nanoparticle may be formed by combining a platinumnanoparticle with a metal salt in a solution. Ions of the second metalreact with platinum and replace platinum atoms on the nanoparticle.Platinum atoms from the edge and corner regions react with the secondmetal ions quicker than platinum surface atoms on the terraces, due to agreater difference in electrode potential between the platinum atoms atthe edge and corner regions, as compared to the second metal in thesolution.

In some embodiments, the platinum nanoparticle may include surfacedefects, such as steps and kinks. In that case, surface defects on thestabilized platinum nanoparticle are formed of the second metal atoms.Similar to the platinum atoms from the edge and corner regions of thenanoparticle, the platinum atoms that form the steps and kinks are morereactive than surface atoms on the terraces. As such, when thenanoparticle is mixed with the metal salt, the platinum step and kinkatoms have a lower electrode potential than the ions of the secondmetal. The platinum step and kink atoms thus react with the second metalions quicker than the platinum surface atoms.

In an exemplary embodiment, the platinum nanoparticle is a cathodecatalyst in an electrochemical cell, such as a fuel cell, and the secondmetal is gold. The gold atoms at the corner and edge regions of thenanoparticle result in a more stable catalyst structure.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic of a representative, existing platinumnanoparticle used, for example, as a catalyst, and having a plurality ofterraces, corners and edges.

FIG. 2 is a schematic of a stabilized platinum nanoparticle having atomsfrom a second metal selectively located on edge and corner regions ofthe nanoparticle in place of platinum atoms from the edge and cornerregions.

FIG. 3 is a block diagram illustrating a method of producing a stableplatinum nanoparticle similar to the nanoparticle of FIG. 2.

FIGS. 4A-4D are schematics illustrating the method of FIG. 3 forselectively replacing the platinum atoms from the edge and cornerregions of the nanoparticle with a second metal.

FIG. 5A is a schematic of an enlarged portion of one of the Pt (100)terraces of the nanoparticle of FIG. 4A to illustrate surface defects,including step atoms and kink atoms, that may exist on the nanoparticle.

FIG. 5B is a schematic of the Pt (100) terrace from FIG. 5A after twoplatinum step atoms and a platinum kink atom have been replaced by twogold atoms.

FIG. 6A is a plot of voltammetry curves comparing standard platinumnanoparticles to platinum nanoparticles exposed to a metal salt in anacid solution for five minutes, in order to compare the electrochemicalactive area of the nanoparticles before and after the reaction.

FIG. 6B is a plot similar to FIG. 6A comparing standard platinumnanoparticles to platinum nanoparticles exposed to a metal salt insolution for twenty minutes.

FIG. 7A is a plot of polarization for the two samples from FIG. 6A inorder to determine the oxygen reduction reaction (ORR) activity levelfor each of the samples.

FIG. 7B is a plot similar to FIG. 7A for the two samples from FIG. 6B.

FIG. 8 is a schematic of a fuel cell that uses the platinumnanoparticles described herein as a stabilized cathode catalyst.

FIGS. 9A-9C are schematics of a cubic-shaped nanoparticle, also suitablefor use as a catalyst, as it undergoes the process of having theplatinum atoms at the edge and corner regions replaced with atoms from asecond metal.

FIGS. 10A-10C are similar to FIGS. 9A-9C and illustrate atetrahedron-shaped nanoparticle undergoing the method for replacing theplatinum atoms at the edge and corner regions of the nanoparticle.

It is noted that the drawings are not to scale.

DETAILED DESCRIPTION

A stabilized platinum nanoparticle is described herein which includes asecond metal (for example, gold) located on select areas of an outersurface of the nanoparticle. A method of producing stabilizednanoparticles is also described below and includes replacing platinumatoms at edge and corner regions of the nanoparticles with atoms of thesecond metal. Platinum atoms that form surface defects on thenanoparticle, such as steps and kinks, may also be replaced with atomsof the second metal. Platinum nanoparticles are commonly used as acatalyst and the nanoparticle structure described herein results in amore stable catalyst. In an exemplary embodiment, the platinumnanoparticles may be used as a cathode catalyst for an oxygen reductionreaction (ORR) in a fuel cell.

Platinum nanoparticles may be produced using known synthesis methods,such as chemical reduction. The platinum nanoparticles may be preparedas colloidal particles, and the size and shape of the nanoparticles maybe controlled based on the conditions during synthesis. In an exemplaryembodiment in which the platinum nanoparticles are used as a catalyst, asuitable range for the diameter of the nanoparticles described herein isbetween approximately 0.5 and 100 nanometers (nm). In some embodiments,the diameter ranges between approximately 1 and 20 nm; in otherembodiments, the diameter ranges between approximately 1 and 10 nm.

FIG. 1 is a schematic of representative, existing nanoparticle 10, whichhas a cubo-octahedron shape. Nanoparticle 10 includes a core or insideportion and outer surfaces 12. In an exemplary embodiment, surfaces 12are formed from a plurality of platinum atoms 14 bonded together tocreate a plurality of flats or terraces 16, edges 18, and corners 19.Each edge 18 represents an intersection of two adjoining terraces 16,and each corner 19 is an intersection of at least three edges 18. In theembodiment shown in FIG. 1, corners 19 represent an intersection ofthree edges 18. Platinum atoms 14 that form terraces 16 are surfaceatoms. For purposes of this disclosure, in a Pt (100) facet or surface,a surface atom is defined as an atom having eight nearest neighboratoms, since platinum has a face-centered cubic unit cell. Surface atomshave a lower surface energy than corner and edge atoms.

In the embodiment shown in FIG. 1, nanoparticle 10 has a regularcubo-octahedron shape, and terraces 16 are essentially flat and free ofdefects. It is recognized that nanoparticle 10 may commonly have a moreirregular shape and terraces 16 may include surface features or defects,such as steps and kinks. These surface defects are described furtherbelow in reference to FIG. 5A.

Although not visible in FIG. 1, the core or inside portion ofnanoparticle 10 may be formed of platinum or a platinum alloy. Othermetals used to form the platinum alloy core may include transitionmetals from periods 4, 5, and 6 of the periodic table. Alternatively,essentially all of the core of nanoparticle 10 may be formed by at leastone metal other than platinum. In the exemplary embodiment of FIG. 1,outer surfaces 12 are formed essentially of platinum atoms 14. Dependingon a composition of the core or inside portion, the platinum atoms thatform outer surfaces 12 may be formed from only one layer of platinumatoms. Alternatively, outer surfaces 12 may be formed from two or morelayers of platinum atoms. In an alternative embodiment, all ofnanoparticle 10, including outer surfaces 12, may be formed of aplatinum alloy.

FIG. 2 is a schematic of stabilized nanoparticle 20, which also has acubo-octahedron shape. The stabilized nanoparticles described herein mayinclude nanoparticles of any known shape and other examples are shown inthe figures and discussed below. Similar to nanoparticle 10,nanoparticle 20 has a core portion and outer surfaces 22, which includeterraces 26, edges 28 and corners 29. Similar to nanoparticle 10, thecore portion of nanoparticle 20 may be formed of platinum, a platinumalloy or at least one non-platinum metal. Terraces 26 are formed ofplatinum atoms 14, also similar to nanoparticle 10. In contrast tonanoparticle 10, edges 28 and corners 29 are formed of second metalatoms 30. In an exemplary embodiment, atoms 30 are gold atoms (Au). Asshown in FIG. 2, gold atoms 30 are larger in size compared to platinumatoms 14; however, the size differential between gold and platinum atomsis exaggerated in FIG. 2. Nanoparticle 20 is approximately the same sizeas nanoparticle 10. Because a portion of nanoparticle 20 is formed ofsecond metal 28, nanoparticle 20 uses less platinum compared tonanoparticle 10. This is beneficial since platinum is an expensivemetal.

As shown in FIG. 2, nanoparticle 20, similar to nanoparticle 10 of FIG.1, has a regular cubo-octahedral shape and is essentially free ofdefects. As such, terraces 26 are formed of essentially all surfaceatoms. It is more common that nanoparticle 20 would have surface defectsand some irregularity in its shape. For example, as described below andshown in FIG. 5A, terraces 26 may have steps that make each terrace 26an irregular surface.

FIG. 3 is a flow diagram illustrating method 40 for producing astabilized platinum nanoparticle, similar to nanoparticle 20 of FIG. 2,by selectively removing platinum atoms from the edge and corner regionsof the nanoparticle and replacing the removed platinum atoms with atomsfrom a second metal. In an exemplary embodiment, the second metal isgold (Au). Other metals may also be used in addition to gold, including,but not limited to, iridium, rhodium, ruthenium, rhenium, osmium,palladium, silver, and combinations thereof. Method 40 includes steps42-56, and begins with obtaining platinum nanoparticles (step 42)similar to nanoparticle 10 of FIG. 1. The nanoparticles may be comprisedessentially of platinum and platinum alloys, and may be of any knownshape, as discussed further below. Step 42 of method 40 may includesynthesis of the platinum nanoparticles using any known method.Alternatively, the obtainment of the nanoparticles in step 42 mayinvolve purchasing the platinum nanoparticles.

A next step in method 40 is to add the platinum nanoparticles into asolution (step 44). In an exemplary embodiment, the solution is anacidic solution, including, but not limited to, sulfuric acid andperchloric acid. Other solutions may include, but are not limited to, analkaline solution and a non-aqueous solution. An example of an alkalinesolution is sodium hydroxide. An example of a non-aqueous solution isethylene glycol. In one embodiment, the platinum nanoparticles may besupported on an electrically conductive substrate, such as, but notlimited to, carbon black, a metal oxide, a metal carbide, boron dopeddiamond, and combinations thereof. In that case, the substrate carryingthe platinum nanoparticles is added to the solution. In an alternativeembodiment, the platinum nanoparticles are unsupported and insteaddispersed in a solution, which is then added to the solution in step 44.

A metal salt, such as, for example gold trichloride (AuCl₃), is thenadded to the solution in step 46. It is recognized that steps 44 and 46may occur in reverse order or occur simultaneously so long as thenanoparticles are combined with the metal salt. Placing the metal saltin the acidic solution forms a solution containing gold ions (Au³⁺).Platinum atoms on a surface of the nanoparticles react with the goldions in a standard oxidation reduction reaction (redox) (step 48):

Pt→Pt²⁺+2e ⁻  (1)

Au³⁺+3e ⁻→Au  (2)

As a result of the reaction in step 48, platinum atoms (Pt) are oxidizedto form platinum ions (Pt²⁺), which then dissolve into the solution. Thegold ions (Au³⁺) in the solution are reduced by the platinum to formgold atoms (Au), which may then replace the platinum atoms on thenanoparticles. The driving force for this reaction is a difference inelectrode potential between gold and platinum in the solution. Thestandard electrode potential of gold is higher than the standardelectrode potential of platinum. Platinum atoms at the corner and edgeregions of the nanoparticles have a lower electrode potential thanplatinum surface atoms on the terraces or flats of the nanoparticles.Thus, the platinum atoms at the corner and edge regions have a muchlower electrode potential relative to the gold ions in the solution. Thelarge difference in electrode potential causes gold ions in the solutionto be reduced by platinum atoms at the corner and edge regions of thenanoparticle. The platinum from the corner and edge regions is oxidizedto form platinum ions. The electron transfer from the platinum atoms tothe gold ions (to form gold atoms) occurs at the corner and edgeregions, and thus the gold atoms replace the platinum atoms at thecorners and edges of the nanoparticle. Due to a difference in valency,three platinum atoms reduce two gold ions, as shown by the equationsbelow:

3Pt→3Pt²⁺+6e ⁻  (3)

2Au³⁺+6e ⁻→2Au  (4)

The reduction of the gold ions to gold atoms by platinum first occurs atthe corner and edge regions of the nanoparticle due to the largerdifference in electrode potential between the gold and the platinumatoms at the corners and edges. Over time, the gold atoms would alsoreplace the platinum surface atoms on the terraces or flats of thenanoparticle. However, the rate of these reactions is slower due to asmaller difference in electrode potential between the gold in thesolution and the platinum surface atoms on the terraces of thenanoparticle. As described below, the nanoparticles are only left in thesolution for a certain period of time, in order to prevent replacementof the platinum surface atoms on the terraces of the nanoparticle.

When the platinum nanoparticles are mixed with the metal salt insolution, the reaction of platinum and gold in step 48 occurs due to adifference in electrode potentials. In some embodiments, step 48 mayinclude stirring the solution to avoid the mass transport effect, and topromote the reaction between platinum and gold. Stirring may beperformed, for example, by a magnetic stirrer. In some embodiments, thesolution may also be heated, using, for example, a burner. Thetemperature of the heated solution may be between approximately 40 and300 degrees Celsius.

In step 50, the platinum nanoparticles are removed from the solutionafter a time determined to be sufficient to replace the platinum atomsessentially only on the edge and corner regions 30 and 32 ofnanoparticle 20, such that terraces 26 remain unchanged. In an exemplaryembodiment, the platinum nanoparticles are removed approximately four tofive minutes after adding the metal salt in step 46. It is recognizedthat this time may increase or decrease depending, in part, on the typeof metal salt, the concentration of the metal salt, the temperature ofthe reaction, and the volume of nanoparticles. The relative reactivityof the platinum atoms at the edges and corners, as well as at any stepsand/or kinks (i.e. surface defects), may also impact the reaction timeto replace the platinum nanoparticles essentially only at the edges,corners and defects of the nanoparticle. For example, the relativereactivity of the platinum atoms at edges and corners may vary as afunction, in part, of an overall shape of the nanoparticles.

In the embodiment in which the nanoparticles are dispersed in solutionand unsupported, the nanoparticles are filtered in step 50 in order toremove the nanoparticles from the solution. Next, in step 52, thenanoparticles are washed with distilled water and then dried. In someembodiments, the nanoparticles may be dried in a vacuum.

An optional step in method 40 is to heat treat the nanoparticles (step54) at approximately 200 to 500 degrees Celsius for approximately 0.5 to2 hours. The heat treatment may also include nitrogen or hydrogen gas,or a mixture of the two. Because hydrogen is a reducing agent, exposingthe platinum nanoparticles to hydrogen under heat may ensure that anygold ions on the nanoparticles that were not completely reduced to goldatoms in the solution, or gold atoms physically adsorbed on the platinumsurface, may be reduced during the heat treatment. The gold atomsgenerally remain on the surface, rather than migrate into a bulk or coreregion of the nanoparticle, due to surface segregation of gold. Duringannealing, gold atoms may tend to move to the edge and corner regions ofplatinum particles, where they are more stable compared to at theterraces.

As described above, the gold atoms first replace the platinum atoms fromthe edge and corner regions of the platinum nanoparticle. So long as thenanoparticles are removed from the gold ions after a predetermined time,the surface atoms on the terraces of the nanoparticle, in general,remain unchanged. It is recognized, however, that some gold atoms maydeposit onto the terraces during the period intended only forreplacement of corner and edge regions. It is believed that heattreating the nanoparticles in step 54 may cause any gold atoms on theterraces to diffuse to the edge and corner regions.

Platinum from the corners and edges of the nanoparticle is oxidized bythe gold ions to form platinum ions, which are dissolved into thesolution. In step 56 of method 40, the dissolved platinum ions may berecycled to synthesize additional platinum nanoparticles. Alternatively,the platinum ions may be recycled for other uses.

In some embodiments, method 40 may include an optional step (not shownin FIG. 3) of blocking the terraces of the nanoparticle using asurfactant. In that case, the surfactant is adsorbed onto the terracesurfaces. This optional step may be performed prior to mixing theplatinum nanoparticles with the second metal (step 44). Because thesurfactant covers the platinum atoms on the terraces of thenanoparticle, the surfactant prevents gold atoms from replacing platinumatoms on the terraces of the nanoparticle. One example of a surfactantthat may be used is polyvinylpyrrolidone (PVP).

Method 40 of FIG. 3 is an electroless deposition process that is basedon a standard oxidation reduction reaction (redox). As described above,gold and other metals, such as iridium, rhodium, ruthenium, rhenium,osmium, palladium and silver, may replace platinum atoms at the cornersand edges, as well as at any surface defects, to form a stabilizedplatinum nanoparticle using method 40. Other methods may be used toproduce a stabilized platinum nanoparticle having a second metal onthese reactive regions of the nanoparticle.

The selected method may depend, in part, on the particular metal beingused to form the stabilized platinum nanoparticle. Other metals inaddition to those disclosed above may also be used to form a stabilizedplatinum nanoparticle by protecting the platinum nanoparticle at edgesand corners, as well as at any surface defects. These additional metalsinclude transition metals from groups four through six of the fourth,fifth and sixth row of the periodic table. The metals which may beincluded in these alternative methods include, but are not limited to,titanium, zirconium, hafnium, vanadium, niobium, tantalum, chromium,molybdenum, and tungsten. These metals may act as strong oxide formersonce deposited onto the platinum nanoparticle. Methods for forming thestabilized nanoparticle using these additional metals include, but arenot limited to, deposition of the transition metals from a complex in asolution or a vapor phase, electrodeposition of the transition metalfrom a solution, chemical reduction by a strong reducing agent, andvapor deposition of ions of the transition metal. These additionalmethods result in a stabilized platinum nanoparticle having a secondmetal that is a strong oxide former. In these additional methods, theformation of the second metal at the edge and corner regions, as well asat any surface defects, may occur, in part, through migration orsegregation.

Additional steps may be taken to enhance the stabilization of thenanoparticle by the transition metal. These additional processing stepsmay include, but are not limited to, an annealing treatment at elevatedtemperatures and a conditioning treatment in a strong oxidizingatmosphere. Moreover, the terraces of the nanoparticle may betemporarily capped with protective ligands, such as sulfate or phosphategroups. This is similar to an optional step described above, undermethod 40, of blocking the terraces using a surfactant.

FIGS. 4A-4D are schematics showing nanoparticle 10 of FIG. 1 undergoingmethod 40 to form nanoparticle 20 of FIG. 2. FIG. 4A shows nanoparticle10, which is initially formed only of platinum atoms 14, exposed to goldions 60 (Au³⁺). As shown in FIG. 4B, three platinum atoms have beenoxidized by gold to form three platinum ions 62 (Pt²⁺) which are thendissolved into the solution. Two gold ions (Au³⁺) are reduced by thethree platinum atoms to form two gold atoms 64, which replace theplatinum atoms on nanoparticle 10. FIG. 4C shows nanoparticle 10 afterplatinum and gold have been together in solution for a longer period oftime and additional gold atoms 64 are shown bonded to the corner andedge regions of nanoparticle 10. It is recognized that the replacementprocess of gold for platinum may be somewhat random in terms of an orderin which the platinum atoms are replaced. In some cases, a platinum atomon a terrace may be replaced before replacement of all the corners andedges; however, in general, the replacement first occurs on the cornerand edge regions of the nanoparticle. Due to the difference in valency,two gold atoms replace three platinum atoms. Because gold is a largeratom compared to platinum, a single gold atom occupies more of thecorner and edge region compared to a single platinum atom.

FIG. 4D is a schematic of nanoparticle 10 after essentially all thecorner and edge regions have been replaced by gold atoms 64; thus,nanoparticle 10 is converted to nanoparticle 20. It is recognized thatnanoparticle 20 may have a slightly irregular shape due to a differencein the total number of atoms before and after method 40, as well as adifference in size between platinum atoms and gold atoms. Moreover, itis recognized that a minimal amount of gold atoms may attach to theflats of nanoparticle 20. However, so long as nanoparticle 20 is removedfrom the solution at a predetermined time, in general, the flats ofnanoparticle 20 should remain unchanged.

In an exemplary embodiment, the metal that replaces the platinum at theedge and corner regions of the nanoparticles is gold (Au). Althoughplatinum is a noble metal, in operation as a catalyst in a fuel cell,platinum atoms on the platinum nanoparticle are unstable and may beoxidized. This causes the platinum atoms to dissolve from thenanoparticle, resulting in an unstable platinum catalyst. Gold is wellsuited for this application because it is a more noble metal compared toplatinum and is less likely to be oxidized during cycling of the fuelcell. By coating the edges and corners of the nanoparticle with gold,the gold does not dissolve during operation of the fuel cell and thecatalyst remains stable over time. Moreover, as described further below,if only the corner and edge regions of the nanoparticle are replacedwith gold, the impact on the ORR activity of the platinum catalyst isnegligible.

Another advantage of using gold in this application is that gold has anoverall higher standard electrode potential than platinum. As describedabove, the driving force of an oxidation reduction reaction is adifference in electrode potential between the oxidant (gold ions) andthe reductant (platinum atoms). Comparing two similarly located atoms inwhich one is gold and one is platinum, gold has a higher standardelectrode potential than platinum. However, when the platinum atom islocated at an edge or a corner region of a nanoparticle, that platinumatom has a higher surface energy and a consequently lower electrodepotential compared to a platinum atom on a terrace of the nanoparticle.As such, the difference in electrode potential between the gold and theplatinum atom at the corner or the edge is even greater. As describedabove, this difference in electrode potential is why the gold atomsreplace the platinum atoms first at the corner and edge regions of thenanoparticle. Although the platinum atoms on the flats of thenanoparticle may still have an electrode potential lower than gold, thedifference in electrode potential is smaller. Therefore, the reactiongenerally does not occur for the platinum atoms on the flats until themore reactive atoms (i.e. at the corners and edges) are replaced.

The goal of method 40 is to replace the platinum atoms with a secondmetal only at the edge and corner regions of the nanoparticle. Othermetals, in addition to gold, may also be used, including, but notlimited to, iridium, rhodium, ruthenium, rhenium, osmium, palladium,silver, and combinations thereof. It is not required that the standardelectrode potential of the second metal is greater than the overallstandard electrode potential of platinum, but rather that the electrodepotential of the metal ions in solution is greater than the electrodepotential of the platinum at the corner and edge regions of thenanoparticles. As such, in some embodiments, the second metal may have astandard electrode potential that is about equal to or even less thanthe standard electrode potential of platinum.

As described above, other methods in addition to method 40 may be usedto form a stabilized platinum nanoparticle. These alternative methodsmay use other metals, such as transition metals that act as strong oxideformers on the nanoparticle.

As shown in FIG. 4D, gold atoms 64 form the edge and corner regions ofnanoparticle 20. In the embodiment shown in FIG. 4D, nanoparticle 20 islarge enough such that the majority of the total surface area ofnanoparticle 20 is still formed by platinum atoms 14. For smaller sizednanoparticles, which are formed of less platinum atoms, the gold atomsthat form the edge and corner regions occupy a greater portion of thetotal surface area of the nanoparticle. As mentioned above, in someembodiments, a suitable range of the diameter of the platinumnanoparticles is between approximately 1 and 20 nm; in otherembodiments, the diameter ranges between approximately 1 and 10 nm. Forsmaller-sized nanoparticles (i.e. less than 1.5 nm), the gold atoms (orother second metal) occupy more of the surface area of the nanoparticle.As such, the gold (or other second metal) may occupy up to approximatelyseventy-five percent of the total surface area of the nanoparticle. Onthe other hand, nanoparticles up to or greater than 10 nanometers mayalso be used, and thus the gold may occupy as little as approximatelyfive percent of the total surface area. Therefore, once the second metalatoms replace the platinum atoms on the surface, the second metal atomsmay occupy between approximately five and approximately seventy-fivepercent of a total surface area of the nanoparticle.

The nanoparticles described herein use less platinum compared tonanoparticle 10 of FIG. 1 because the gold atoms replace platinum atomson the nanoparticles. The replaced platinum may then be recycled. Gold(or another second metal) selectively replaces platinum at the cornersand edges, as well as at any surface defects, based on the difference inelectrode potential. Because the gold atoms only cover the corners andedges, and any surface defects, the nanoparticles maintain theircatalytic activity, but are more durable during potential cycling.

The nanoparticles shown thus far have had regular cubo-octahedron shapesand have been essentially free of defects. As such, the terraces of thenanoparticles have been shown as flat surfaces comprised essentially ofall surface atoms. As stated above, in reality, the nanoparticlesdescribed herein commonly have surface defects that form as a result ofthe synthesis process used in forming the nanoparticles. FIG. 5A is aschematic of an enlarged portion of one of terraces 16 from nanoparticle10 of FIG. 4A to illustrate these surface defects. Terrace 16 of FIG. 5Ais a (100) surface, and thus is referred to as Pt (100) terrace 16.FIGS. 5A and 5B illustrate that, if these surface defects are present ona nanoparticle, gold atoms may likely replace some of the platinum atomsat the surface defects. This occurs because the platinum atoms that formthe surface defects are more reactive sites on the nanoparticle, similarto edge and corner atoms. (Note that the surface defects shown in FIG.5A are not visible in FIG. 4A.)

As shown in FIG. 5A, Pt (100) terrace 16 is formed of all platinum atoms14 and includes stable portion 16 a and ledge 66. Ledge 66 is a layer ofplatinum atoms 14 that forms over part of stable portion 16 a, resultingin an elevated layer of atoms 14. Similar to atoms 14 on stable portion16 a, the majority of atoms 14 on ledge 16 are surface atoms. Becauseplatinum has a face-centered cubic unit cell, a surface atom on a (100)surface has eight nearest neighbor atoms. The stability of each atom isa function, in part, of how many other atoms are surrounding that atom.Like the surface atoms in stable portion 16 a, most atoms on ledge 16have eight nearest neighbor atoms. However, platinum atoms 14 located ina last row of ledge 66 (labeled as 66 a) are more reactive because theseatoms have no more than seven nearest neighbor atoms. More specifically,last row 66 a includes step atoms 67, kink atom 68 and step adatom 69.Step atoms 67 are defined as atoms having seven nearest neighbor atoms.Kink atom 68 has six nearest neighbor atoms, including a step atom 67.Finally, step adatom 69 has only four nearest neighbor atoms. It isrecognized that the nearest neighbor atoms for surface atoms, stepatoms, kink atoms and step adatoms may vary based on thecrystallographic orientation of the facet surface.

FIG. 5A shows gold ions 60 (Au³⁺) near terrace 16. As described above,gold ions 60 react with platinum atoms 14 in an oxygen reductionreaction, and as a result, gold atoms may replace platinum atoms on thenanoparticle. This reaction is driven by a difference in electrodepotential between the platinum atoms on the nanoparticle and the goldions in solution. The difference in electrode potential between atoms67, 68, 69 and gold ions 60 in solution is much greater than thedifference between platinum surface atoms and gold ions 60. Thus, atoms67, 68 and 69, like the corner and edge atoms, react with gold ions 60quicker than platinum surface atoms on stable portion 16 a.

FIG. 5B shows Pt (100) terrace 16 after two of gold ions 60 (Au³⁺) havereacted with three platinum atoms 14 to form two gold atoms 64 on thenanoparticle, and three platinum ions 62 (Pt²⁺) dissolve into solution.As shown in FIG. 5B, gold atoms 64 replaced step adatom 69, kink atom 68and one step atom 67. Step adatom 69 and kink atom 68 are replaced bygold atoms 64 quicker than other platinum atoms on ledge 66 due to ahigher level of reactivity and a lower electrode potential, relative togold. It is recognized that multiple ledges and steps may be present onterrace 16, including multiple ledges on top of one another. Thenanoparticles described herein may vary in terms of an amount of surfacedefects present on the nanoparticles.

As described above, the platinum nanoparticles are removed from themetal salt solution after a time sufficient such that the terraces,which are the less-reactive regions of the nanoparticles, remainunchanged. More specifically, the platinum surface atoms on the terracesdo not react with the second metal due to a smaller difference inelectrode potential. By contrast, the atoms that form the steps andkinks on the terraces may likely be replaced with the second metalatoms, because these atoms are more reactive than surface atoms on theterraces. Unless a nanoparticle has an unusually large number of surfacedefects, the majority of the terraces should remain unchanged so long asthe nanoparticles are removed from the solution after a time determinedsufficient to only replace the platinum atoms at the reactive sites onthe nanoparticle. Depending on an amount of surface defects, the secondmetal atoms may occupy a greater percentage of the surface area of thenanoparticle than the ranges provided above, which were based on theedge and corner regions of the nanoparticle.

FIG. 6A is a plot of a cyclic voltammetry curve comparing existingplatinum nanoparticles (like nanoparticle 10 of FIG. 1 and designated assample 1 in FIG. 6A) to a stabilized platinum nanoparticle describedherein (like nanoparticle 20 of FIG. 2 and designated as Sample 2 inFIG. 6A). The nanoparticles of sample 2 were kept in solution with ametal salt (AuCl₃) for approximately five minutes. In both samples 1 and2, the nanoparticles were generally cubo-octahedral shaped nanoparticleshaving a diameter of approximately five nanometers. It is recognizedthat samples 1 and 2 may include nanoparticles having other shapes inaddition to cubo-octahedral nanoparticles. The other shapes may include,for example, generally spherical or quasi-spherical nanoparticles andother irregular shapes. The nanoparticles in samples 1 and 2 may alsohave surface defects which may contribute to an irregular shape of thenanoparticles.

The electrochemical active area (ECA) of a platinum catalyst iscalculated based on the hydrogen adsorption charge. Comparing the valuesfor hydrogen adsorption between samples 1 and 2, the ECA of thenanoparticles of sample 2 decreased by approximately 18 percent comparedto the ECA for the nanoparticles of sample 1. This suggests thatapproximately 18 percent of a surface area of the nanoparticles insample 2 was replaced by gold. For a cubo-octahedral shaped nanoparticlehaving a diameter of approximately five nanometers, the corner and edgeatoms account for approximately 18 percent of the total surface atoms.Thus, the plot in FIG. 6A supports a conclusion that, afterapproximately five minutes in the solution containing the metal salt,the atoms at the corner and edge regions of the nanoparticles arereplaced with atoms from the second metal, while the terraces of thenanoparticles remain generally unchanged.

FIG. 6B is a voltammetry curve similar to FIG. 6A for samples 3 and 4.Sample 3, similar to sample 1, includes standard platinum nanoparticlesthat are generally cubo-octahedral shaped and have a diameter ofapproximately 5 nanometers. Sample 4 is similar to sample 2 of FIG. 6A,but the nanoparticles of sample 4 were kept in the solution with themetal salt for approximately 20 minutes. As stated above in reference tosamples 1 and 2, the nanoparticles in samples 3 and 4, althoughgenerally cubo-octahedral shaped, may include other shaped nanoparticlesand nanoparticles having surface defects.

Comparing the values for hydrogen adsorption between samples 3 and 4,the ECA of the nanoparticles of sample 4 decreased by approximately 22percent compared to the ECA for the nanoparticles of sample 3. Theseresults indicate that after a longer period of time the gold atoms fromthe metal salt begin to also replace platinum atoms from the terraces ofthe nanoparticle. The displacement of the platinum atoms from theterraces is slower because these atoms are less reactive, due to asmaller difference in potential between the metal ions in the solutionand the platinum atoms on the terraces or flats of the nanoparticle. Theresults from FIGS. 6A and 6B show that replacing the platinum atomsessentially only on the corner and edge regions of the nanoparticles maybe controlled by controlling the amount of time that the nanoparticlesare in contact with the second metal solution.

FIG. 7A is a plot of polarization for samples 1 and 2 from FIG. 6A, andcompares the oxygen reduction reaction (ORR) activity for the twosamples. At a voltage equal to 0.9 V, as shown in FIG. 7A, the currentdensity for sample 2 is 5.82 mA cm⁻², whereas the current density forsample 1 is 5.97 mA cm⁻². The mass activity (not shown in FIG. 7A) at0.9 V is 0.22 A mg⁻¹ _(Pt) for sample 1 and 0.20 A mg⁻¹ _(Pt) for sample2. The reduction in current density and mass activity for sample 2, ascompared to sample 1, is due to a loss in the total amount of platinumon the nanoparticles of sample 2, thus reducing the catalytic activityof the nanoparticles. However, the loss of ORR activity in sample 2,relative to sample 1, is negligible. On the other hand, the specificactivity of the nanoparticles in sample 2 is calculated to be 0.54 mAcm⁻², compared to a value of 0.51 mA cm⁻² for the sample 1nanoparticles. The specific activity is the kinetic current densitynormalized to the ECA. The increase in specific activity for sample 2,compared to sample 1, is reasonable since the platinum atoms at thecorners and edges of the nanoparticle are typically less active for theORR, compared to the atoms on the flats of the nanoparticle.

FIG. 7B is a plot of polarization similar to FIG. 7A comparing samples 3and 4. The nanoparticles of sample 4 were left in the metal saltsolution for approximately 20 minutes. As shown by FIG. 7B, thedifference in ORR activity between samples 3 and 4 is greater than thedifference between samples 1 and 2 of FIG. 7A, especially in the higheroverpotential region (i.e. below approximately 0.9 V). The currentdensity of sample 4 is 5.61 mA cm⁻¹, compared to a value of 5.97 mA cm⁻¹in sample 3. The results of FIG. 6B indicate that between approximately5 minutes and 20 minutes in the metal salt solution, the platinum atomson the terraces of the nanoparticles begin to be replaced by gold atoms.FIG. 7B illustrates that even if a small portion of the platinum atomsfrom the flats are replaced with gold atoms, the ORR activity decreasessignificantly, and the nanoparticles of sample 4 are less effective asan electrocatalyst, compared to those of sample 2.

Platinum nanoparticles are commonly used as an electrocatalyst in anelectrochemical cell, and the stabilized platinum nanoparticlesdescribed herein may result in a more active catalyst. FIG. 8 is anexemplary embodiment of fuel cell 70, which includes the platinumnanoparticles described herein as a stabilized cathode catalyst layer.

Fuel cell 70 is designed for generating electrical energy and includesanode 72, anode catalyst layer 74, electrolyte 76, cathode 78, andcathode catalyst layer 80. Anode 72 includes flow field 82 and cathode78 includes flow field 84. In an exemplary embodiment, fuel cell 70 is ahydrogen cell using hydrogen as fuel and oxygen as oxidant. It isrecognized that other types of fuels and oxidants may be used in fuelcell 70.

Anode 72 receives hydrogen gas (H₂) by way of flow field 82. Catalystlayer 74, which may be a platinum catalyst, causes the hydrogenmolecules to split into protons (H⁺) and electrons (e⁻). Electrolyte 76allows the protons to pass through to cathode 78, but the electrons areforced to travel to external circuit 86, resulting in a production ofelectrical power. Air or pure oxygen (O₂) is supplied to cathode 78through flow field 84. At cathode catalyst layer 80, oxygen moleculesreact with the protons from anode 72 to form water (H₂O), which thenexits fuel cell 70, along with excess heat.

Anode catalyst layer 74 and cathode catalyst layer 80 may each be formedfrom platinum nanoparticles. As described above, cathode catalyst layer80 is used to increase the rate of the oxygen reduction reaction (ORR)causing the formation of water from protons and oxygen. Even thoughplatinum is a catalytic material, the platinum is unstable in thisenvironment. During potential cycling, platinum atoms from the platinumnanoparticles dissolve, particularly starting from corner and edgeregions of the nanoparticles. The platinum nanoparticle described hereinand shown in FIGS. 2 and 4D is a more stable nanoparticle and is moredurable for use as cathode catalyst layer 80. It is recognized that thenanoparticle of the present invention may also be used for anodecatalyst layer 74. However, the platinum is more stable in theenvironment used for anode catalyst layer 74, and the problems describedabove for cathode catalyst layer 80 do not generally apply to layer 74.

In one embodiment, fuel cell 70 is a polymer electrolyte membrane (PEM)fuel cell, in which case electrolyte 76 is a proton exchange membraneformed from a solid polymer. In an alternative embodiment, fuel cell 70is a phosphoric acid fuel cell, and electrolyte 76 is liquid phosphoricacid, which is typically held within a ceramic matrix. Cubo-octahedralshaped nanoparticles, like nanoparticle 10 of FIG. 1, are commonly usedin fuel cells, including PEM and phosphoric acid fuel cells. Othernanoparticle shapes that have been studied for use as platinum catalystsinclude, but are not limited to, cubic and tetrahedral nanoparticles.Specific shaped nanoparticles may be more stable in a specific type ofelectrolyte.

It is recognized that a platinum catalyst may use nanoparticles having avariety of shapes and it is not required that a particular shape be usedwith a particular fuel cell. However, the ORR activity may beinfluenced, in part, by a combination of the type of electrolyte and theshape of the nanoparticles. This may be due to a difference in thecrystal faces that form the shapes of the nanoparticles. Cubicnanoparticles are formed essentially of all (100) surfaces, whereastetrahedral nanoparticles are formed of (111) surfaces.

In some embodiments, the cathode catalyst for a phosphoric acid fuelcell is formed of cubic-shaped platinum nanoparticles having corner andedge regions formed of a second metal (see FIG. 9C). If the cubicnanoparticles include a second metal, such as gold, the cathode catalystis more stable, and thus should have a longer operational life since atotal mass of the cathode catalyst should remain relatively constant. Onthe other hand, in a PEM fuel cell, the cathode catalyst, in someembodiments, is formed of tetrahedral-shaped platinum nanoparticleshaving corner and edge regions formed of a second metal (see FIG. 10C).By using a tetrahedral-shaped nanoparticle coated with gold at thecorners and edges, the ORR activity of the nanoparticles in the PEM fuelcell may be further increased. It is recognized that, in either a PEM ora phosphoric acid fuel cell, the catalyst may also include nanoparticlesof at least one other shape.

FIG. 9A is a schematic of nanoparticle 90, which is similar tonanoparticle 10 of FIG. 1, but is cubic-shaped. Nanoparticle 90 has acore formed of platinum, a platinum alloy, or at least one othertransition metal. Outer surfaces 92 of nanoparticle 90 are formed ofplatinum atoms 94, similar to nanoparticle 10. Surfaces 92 are formed offlats or terraces 96, edges 98 and corners 99. Nanoparticle 90, as shownin FIG. 9A, has a regular cubic shape, and terraces 96 are generallyfree of defects. However, as described above, it is recognized thatnanoparticle 90 may commonly have surface defects or features thatresult in nanoparticle 90 having an irregular shape and terraces 96having uneven surfaces. FIGS. 9A-9C illustrate nanoparticle 90 as itundergoes method 40 of FIG. 3 in order to replace platinum atoms 94 atedges 98 and corners 99 with atoms from a second metal.

Nanoparticle 90 of FIG. 9A is combined with a metal salt (for example,AuCl₃) in a solution, as described above, resulting in the formation ofgold ions 100 (Au³⁺). As described above, a difference in electrodepotential between platinum atoms 94 on nanoparticle 90 and gold ions 100in solution drives the redox reaction. Platinum atoms from edges 98 andcorners 99 of nanoparticle 90 dissolve to form platinum ions 102 (Pt²⁺).Gold ions 100 are reduced to form gold atoms 104, which replace platinumatoms on nanoparticle 90. The atoms from edges 98 and corners 99 areoxidized before terrace atoms due to a lower electrode potential ofatoms 94 at edges 98 and corners 99. FIG. 9B shows nanoparticle 90 afterplatinum atoms have dissolved edges 98 and corners 99, and gold atoms104 have begun to replace the platinum atoms, resulting in an irregularshape for nanoparticle 90.

In FIG. 9C, gold atoms 104 have attached to nanoparticle 90 to formstabilized cubic nanoparticle 110. Although gold atoms are shown in thisexemplary embodiment, other metals in addition to gold may be used asthe second metal. As described above in reference to FIG. 2, gold atomsare larger in size than platinum atoms, although the difference in sizeis further exaggerated in FIGS. 9B and 9C. An overall size ofnanoparticle 110 remains unchanged compared to nanoparticle 90,particularly since three platinum atoms are dissolved for every two goldatoms deposited onto nanoparticle 110. It is recognized thatnanoparticle 110 may have an irregular shape compared to nanoparticle90; however, nanoparticle 110 remains generally cubic-shaped. Althoughnot shown in FIG. 9C, it is recognized that nanoparticle 110 maycommonly include surface defects, such as steps and kinks, on terraces96. The platinum atoms that form the steps and kinks are reactive,similar to edge and corner atoms. As such, the platinum atoms at thesteps and kinks may also be replaced with gold atoms.

FIG. 10A is a schematic of nanoparticle 120, similar to nanoparticles 10and 90, but having a tetrahedron shape. Nanoparticle 120 is formed of acore portion and outer surfaces 122 formed of platinum atoms 124 andincluding terraces 126, edges 128 and corners 129. As stated above inreference to cubic-shaped nanoparticle 90, it is recognized thatnanoparticle 120, in reality, may have a more irregulartetrahedron-based shaped, and that surface defects (i.e. steps andkinks) may commonly be present on terraces 126. FIGS. 10A-10C aresimilar to FIGS. 9A-9C and illustrate the process of forming astabilized tetrahedral shaped nanoparticle.

When nanoparticle 120 is combined with gold trichloride in solution,gold ions 130 (Au³⁺) form, as shown in FIG. 10A. Due to a difference inelectrode potential, platinum atoms 124 from nanoparticle 120 transferelectrons to reduce gold ions 130 to gold atoms 134, which replaceplatinum atoms 124 on nanoparticle 120. The oxidized platinum ions 132(Pt²⁺) are then dissolved into the solution. Because platinum atoms 124at edges 128 and corners 129 are more reactive than platinum atoms 124at terraces 126, gold atoms 134 attach first to the edges and corners ofnanoparticle 120.

FIG. 10C shows stabilized nanoparticle 140 after gold atoms 134 havereplaced essentially all of platinum atoms 124 at edge and cornerregions 128 and 129. Nanoparticle 140 has an irregular shape due to adifference in size between platinum atoms 124 and gold atoms 134, aswell as a difference between the number of platinum atoms removed andthe number of second metal atoms deposited on nanoparticle 140. Sincenanoparticle 120 of FIG. 9A normally would have surface defects onterraces 126, it is recognized that nanoparticle 140 would also includegold atoms on terraces 126 that had replaced any platinum step atoms andkink atoms that had formed the surface defects of nanoparticle 120.

The present disclosure of forming a more stable platinum nanoparticleapplies to all nanoparticles, regardless of shape. Although specificshapes (i.e. cubo-octahedral, cubic and tetrahedral) are described aboveand illustrated in the figures, it is recognized that nanoparticleshaving additional shapes are within the scope of the present disclosure.Other nanoparticle shapes include, but are not limited to, icoshedral,rhombohedral, and other types of polyhedrons. Additional nanoparticleshapes include cylindrical, spherical, and quasi-spherical, which do nothave well-defined edge and corner regions. The atoms that form thedefects, steps and kinks on these nanoparticles are believed to havesimilar reactivity to edge and corner atoms, and these atoms mayconsequently be replaced by the second metal.

Although the stabilized platinum nanoparticles of the present disclosureare described in the context of use as a catalyst in a fuel cell, thenanoparticles may also be used in other types of electrochemical cells,including but not limited to, batteries and electrolysis cells. Thenanoparticles may also be used in other applications that would benefitfrom platinum nanoparticles have a more stable structure, includingother catalyst applications, as well as non-catalyst applications.

Although the present invention has been described with reference topreferred embodiments, workers skilled in the art will recognize thatchanges may be made in form and detail without departing from the spiritand scope of the invention.

1. A nanoparticle having a core portion surrounded by a plurality ofouter surfaces, the nanoparticle comprising: a plurality of platinumatoms that form a plurality of terraces on the nanoparticle; a pluralityof atoms from a second metal that form edge regions and corner regionsof the nanoparticle, wherein the outer surfaces of the nanoparticleinclude the terraces, the edge regions, and the corner regions.
 2. Thenanoparticle of claim 1 wherein the outer surfaces of the nanoparticlefurther include surface defects on the terraces, and the surface defectsare formed of atoms from the second metal.
 3. The nanoparticle of claim2 wherein the surface defects include at least one of a step atom, akink atom, and a step adatom.
 4. The nanoparticle of claim 1 wherein thenanoparticle is selected from a group consisting of cubic nanoparticles,cubo-octahedral nanoparticles, tetrahedral nanoparticles, icoshedralnanoparticles, spherical nanoparticles, quasi-spherical nanoparticles,rhombohedral nanoparticles, and truncated octahedral nanoparticles. 5.The nanoparticle of claim 1 wherein the second metal includes at leastone of gold, iridium, rhodium, ruthenium, rhenium, osmium, palladium,and silver.
 6. The nanoparticle of claim 1 wherein the second metalatoms occupy between approximately 5 and approximately 75 percent of atotal surface area of the outer surfaces of the nanoparticle.
 7. Thenanoparticle of claim 1 wherein the second metal has a standardelectrode potential greater than a standard electrode potential ofplatinum.
 8. The nanoparticle of claim 1 wherein the second metal has astandard electrode potential about equal to or less than a standardelectrode potential of platinum.
 9. The nanoparticle of claim 1 whereinthe core portion of the nanoparticle is formed of at least one ofplatinum, a platinum alloy, and a transition metal selected from a groupconsisting of transition metals from a fourth, fifth, and sixth row ofthe periodic table.
 10. (canceled)
 11. A nanoparticle comprising: a coreformed of at least one of platinum, a platinum alloy and a transitionmetal selected from a group consisting of transition metals from afourth, fifth and sixth row of the periodic table; a plurality ofadjoining terraces surrounding the core, wherein each terrace is formedof platinum surface atoms; a plurality of edge regions formed of atomsfrom a second metal, wherein each edge region is an intersection betweentwo adjoining terraces; and a plurality of corner regions formed ofatoms from the second metal and defining an intersection of at least twoedge regions.
 12. The nanoparticle of claim 11 wherein each terraceincludes surface defects formed of atoms from the second metal.
 13. Thenanoparticle of claim 12 wherein the surface defects include at leastone of a step atom, a kink atom, and a step adatom. 14-16. (canceled)17. The nanoparticle of claim 11 wherein a diameter of the nanoparticleis between approximately 0.5 and approximately 100 nanometers.
 18. Thenanoparticle of claim 11 wherein a diameter of the nanoparticle isbetween approximately 1 and approximately 20 nanometers. 19-20.(canceled)
 21. A stabilized platinum catalyst comprising: a plurality ofnanoparticles, wherein each nanoparticle comprises: a core formed of atleast one of platinum, a platinum alloy and a transition metal selectedfrom a group consisting of transition metals from a fourth, fifth andsixth row of the periodic table; a plurality of adjoining terracesformed of platinum surface atoms; a plurality of edge regions formed ofatoms from a second metal, wherein each edge region is an intersectionof two terraces; and a plurality of corner regions formed of atoms fromthe second metal, wherein each corner region is an intersection of atleast two edge regions; and wherein the core is surrounded by outersurfaces formed of the terraces, the edge regions and the cornerregions.
 22. The stabilized platinum catalyst of claim 21 wherein eachterrace includes at least one of a step atom, a kink atom and a stepadatom formed from the second metal.
 23. The stabilized platinumcatalyst of claim 21 wherein the second metal includes at least one ofgold, iridium, rhodium, ruthenium, rhenium, osmium, palladium, andsilver.
 24. (canceled)
 25. The stabilized platinum catalyst of claim 21wherein the nanoparticles are supported on a substrate.
 26. (canceled)27. The stabilized platinum catalyst of claim 21 wherein the catalyst isan electrocatalyst in an electrochemical cell. 28-30. (canceled)
 31. Thestabilized platinum catalyst of claim 21 wherein a diameter of thenanoparticle is between approximately 0.5 and 100 nanometers. 32.(canceled)